The invention relates to a method of melting a metal especially cast iron, in cupola furnaces operated without coke and an apparatus for carrying out the method. A known cupola furnace operated without coke has a vertically upright casing which is lined internally with a lining of refractory material and contains in its lower part a grate which is provided with a bed of loose-packed refractory bodies, and burners are arranged below the grate and at a distance from it.
Cupola furnaces operated without coke for the melting of metals have been known for decades (DE-B-11 02 977; DE-B-12 43 826; "Giesserei", Volume 74, no. 17, Aug. 17, 1987, pages 493 to 497), the energy required for the melting is produced chiefly with gaseous or liquid fuels. This has numerous advantages compared with the coke firing of cupola furnaces.
As a result of the omission of bed coke and charge coke, a better furnace charge is obtained and work with the slag is considerably facilitated. Temperature control in the furnace can be simpler, that is more rapid and more accurate. Dust removal from and cleaning of the exhaust gases, which with coke-fired furnaces require a large expenditure on apparatus and operation, are reduced to a small amount. The difficulties which occur with the coke-fired furnace because of the sulphur content of the coke, do not exist. Furthermore the thermal efficiency, which for coke-fired cupola furnaces is about 30%, is considerably greater, at up to 70 to 75%. Although processes for cokeless melting have been part of the prior art for a long time and should preferably be used because of continually growing environmental awareness, the ever stricter conditions relating to this and also for reasons of efficient production, it has not previously been possible to develop them to the extent that they satisfy the requirements for foundry products which are customary today. Satisfactory results with regard to reproducible and constant quality characteristics of the melt product have not been achievable hitherto.
An improvement of the process technology or a removal of the known disadvantages of cupola furnaces operated without coke have hitherto only been sought in the field of apparatus, thus, for example, by downstream core-type induction furnaces (DE-A-32 21 241) or by a widening of the furnace hearth with bevelling for enlargement of the radiating surface (DE-B-37 42 349) or by leading the fuel gas through lines laid in heated parts of the furnace lining (DE-B-15 83 279).
The known efforts with the object of process and product optimization led finally only to apparatus measures, with the result of improved energy use of the fuels, but not to a guarantee of a constant melt product with constant quality characteristics. In particular, neither a uniform form of graphite, preferably as A-graphite, nor a homogeneous metal structure has been achieved.
The above cited examples of the prior art show a lively endeavour, but it has never been attempted to solve the existing problems by process changes, for example, by treating the melt directly and early with agents which are used in the cupola furnace itself during the liquefaction processes and before the run-off or before the collection of the molten material in the furnace hearth. Such quality-improving measures are part of the prior art of the coke-fired cupola furnace, for which, to be sure, quite different conditions exist with regard to process engineering, chemistry and metallurgy.
Thus in FR-A-1 226 487 and DE-C-23 29 772 processes are described for exclusively coke-fired cupola furnaces, according to which on the one hand quality improvements and on the other hand the stabilization of desired quality standards are achieved by injecting specific substances into the cupola furnace in the region of the melt zone, that is to say in the zone of the furnace in which the metal melts on the glowing coke and runs off. Regarding this, a process is described in the above cited DE-C-23 29 772 for refining, for nitrogenizing, and for carburization or decarburization, whose use enables defined cast grades to be melted by corrective additives in the form of chemical substances. Serving as such additives are inter alia nitrogen- and/or halogen-releasing compounds in liquid or gaseous form and also, in case carburization is desired, compounds from the distillation of products of vegetable origin and, in case decarburization is desired, such compounds as release oxygen at melt temperature.
By contrast, it has never been attempted to use such a mode of operation also for the cupola furnace operated without coke. Such an idea was in fact opposed among other things because the melt zone, which certainly lies only above the bed of refractory bodies resting on the grate, is much too small in height (about 16 cm) for the added substances to be able to exhibit their effect. For the known processes it has always been emphasized that the added substances must react in the melt flowing off on the glowing coke, and therefore immediately after the liquefaction of the metal.